1. Field of the Invention
The invention relates to a composite multilayer material, in particular for plain bearings or bushings, having a backing layer, a bearing metal layer of a copper alloy or an aluminum alloy, a nickel intermediate layer and an overlay. In addition, the invention relates to uses for the composite multilayer material.
2. Related Art
Conventional composite multilayer materials with the structure comprising steel backing as backing layer, lead-bronze as bearing metal layer and overlay of lead-tin-copper, as described for example in Glyco-Ingenieurberichte 1/91 (Glyco Engineering Reports 1/91), have proven themselves as a result of their high reliability and mechanical load carrying capacity. In such a structure, the overlay is electrodeposited. Such an overlay is a multifunctional layer, in which foreign particles may be embedded, which serves as corrosion protection, which exhibits emergency running characteristics and which is particularly suitable for running-in or mutual conforming of the sliding partners.
The bearing metal layer also exhibits adequate emergency running characteristics, in case the overlay is completely worn away at least in places.
Conventional composite multilayer materials comprise a lead-based overlay, a common alloy being for example PbSn10Cu2. Such overlays exhibit low hardnesses of around 12-15 HV (Vickers Hardness), for which reason they have good embedding properties and are insensitive to seizure.
For reasons of industrial safety and environmental protection, it is nonetheless desirable to replace the lead, which is a heavy metal, with other suitable materials.
One approach is to use hard layers as overlays in heavily loaded bearing systems. For example, aluminum-tin layers with hardnesses of around 80 HV are deposited using PVD (physical vapor deposition) methods. These are lead-free, but are very expensive to produce. Such bearings are highly wear-resistant, but they exhibit almost no embeddability and are therefore generally combined with soft, lead-containing layers as a counter-shell. However, it is also desirable to replace the lead in counter-shells with other materials.
Attempts have been made to use pure tin as a sliding surface. With a hardness of roughly 10 HV, however, pure tin is even softer than the conventional lead alloys and is therefore incapable of absorbing the loads which arise for example in crankshaft main bearings and connecting rod bearings.
DE 197 28 777 A1 describes a composite multilayer material for sliding elements, the overlay of which consists of a lead-free alloy comprising tin and copper, wherein the copper content amounts to 3-20 wt. % and the tin content to 70-97 wt. %. The addition of copper increases the hardness of the overlay relative to pure tin. This overlay is electrodeposited by means of a methylsulfonic acid electrolyte with grain refining additives. The overlay produced in this way has the characteristics of ternary lead-based overlays. In order further to improve wear resistance, DE 197 28 777 A1 additionally proposes providing hard material particles dispersed in the electrolyte bath, these being incorporated into the layer. However, this is associated with additional effort and cost. Between the bearing metal and the overlay it is possible to provide a 1-3 μm thick nickel layer together with a 2-10 μm thick nickel-tin layer as diffusion barrier layer.
DE 197 54 221 A1 discloses a composite multilayer material with an overlay comprising 3-30 wt. % copper, 60-97 wt. % tin and 0.5-10 wt. % cobalt. In this way, the mechanical load carrying capacity is further increased and embrittlement of the bonding layer between overlay and nickel diffusion barrier layer is prevented. The cobalt reduces the tendency of the tin to diffuse towards the nickel. The addition of cobalt to the alloy, however, makes the electrodeposition process more complex, which reduces process reliability. Furthermore, as in DE 197 28 777 A1 the 1-3 μm thick nickel layer may be combined with a 2-10 μm thick nickel-tin layer as diffusion barrier.
EP 1 113 180 A2 describes a composite multilayer material for plain bearings, whose overlay has a tin matrix into which tin-copper particles are incorporated, said particles consisting of 39-55 wt. % copper with the rest being tin. A characteristic feature of the composite multilayer material is, in addition, that not only is an intermediate layer of nickel of a thickness of 1-4 μm provided, but also a second 2-7 μm thick intermediate layer of tin and nickel is arranged between the nickel intermediate layer and the overlay. The first intermediate layer of nickel is applied onto the bearing metal layer from a Watt's nickel electrolyte. A second intermediate layer consisting of nickel and tin is electroplated onto this first intermediate layer. A modified chloride/fluoride electrolyte is used for this purpose. By means of the intermediate layers of nickel and tin-nickel, a system is produced which adapts itself to the load applied, load carrying capacity being increased, according to thermal conditions, by growth of the tin-nickel layer.
The object of the present invention is to overcome the disadvantages of the prior art.